I'm a native English speaker, though whether I have an accent depends on your perspective! I can do either American or British somewhat convincingly, and I usually don't have any trouble being understood by Siri.

I suspect the problem with "50" is that whatever ML underpins it is heavily tuned/biased towards "15" because a lot of people probably set 15 minute timers all the time. 50 minute timers are much less common, I suspect?

Americans think I sound British. British think I sound American. Siri fails less often when set to British instead of American for me, but still fails too often regardless.